Generally, the interfering-type interferometer developed in the laboratory is constructed on the Mach-Zehnder interferometer and the transmission paths thereof are substituted by the optical fibers. When there exists an external perturbation, such as the variation of temperature or pressure, it will cause a variation of a phase retardation (Δφ), namely Δφ(T) or Δφ(P), and appear a variation in an interference-intensity distribution. This is the basic mechanism for measuring the interference.
When the interferometer is constructed by employing the optical fibers, there are two ways: one is using two single-mode fibers, and the other is only utilizing one dual-mode fiber. The most obvious advantages for using only one fiber are small volume, deflectable, and high stability. In the interferometer employing single dual-mode optical fiber, the signal beam and the reference beam in the prior arts are respectively substituted by a fundamental mode and a second-order mode. Thus, the two modes will transmit in one optical fiber and travel an identical distance, and even if the coherent length of a light source is shorter, the interference will not be influenced. On the contrary, in the two-fiber interferometer, it needs to consider the coherent length of the light source, namely the difference of the traveling routes of the beams in two fibers should be within the coherent length, so that the limitations of the optical paths and the spectrum characteristic are increased.
The interferometer which is constructed on the dual-mode optical fiber utilizes the fundamental mode to interfere with a second-order mode. Because the second-order mode group has four eigenmodes and is hard to be excited with a single second-order mode, it causes an unstable interference pattern. Thus, an e-core optical fiber has been proposed. The dual-mode optical fiber in this structure includes the fundamental mode (LP01) and the second-order mode group (LP11), as shown in FIG. 1. In the general o-core (circular-core) fiber, the second-order mode group cannot be separated easily. But, in this e-core fiber, a second-order even mode (LP11even) and a second-order odd mode (LP11odd) have different cutoff wavelengths. Thus, through selecting an appropriate wavelength, these two can be separated, and the fundamental mode and the second-order even mode can be excited sufficiently. Also, through the different phase retardation (Δφ) between the fundamental mode and the second-order even mode, the output interference pattern will be appeared in a different way (the theory is shown in FIG. 2). Basically, the output pattern is two lobes which will mutually rise and fall in response to the difference of the phase retardation, and furthermore, through measuring the contrast intensity of the two lobes, the phase retardation can be quantitated. In this structure, the main difficulties are that the e-core fiber is expensive, and the polarized direction of the incident light should be aligned with the major (or minor) axis of the e-core, or it will cause the propagated light to have an elliptical polarization so as to reduce the visibility of the variation of the interference pattern from the superposition of the fundamental mode and the second-order even mode. Thus, it includes the defect of aligning hardly. Please again refer to FIG. 2 illustrating the theory, one can find that the visibility of variation of the two-lobe pattern is also relative to the energy ratio of the fundamental mode and the second-order even mode. If the energy of the two modes can be distributed appropriately, when the phase retardation (Δφ)=0 and π, one lobe will totally be destroyed and the other will be completely constructed. Thus, according to the variation of the phase retardation, the rise and fall of the two lobes can show an optimal contrast variation. However, because the excited energy of the two modes in the e-core fiber cannot be controlled easily, the visibility is also hard to control.
Because of the technical defects described above, the applicant keeps on carving unflaggingly to develop “a dual-mode fiber-optic interferometer with circular-core fibers and birefringent modal filters and an interfering method thereof” through wholehearted experience and research.